HardSQL challenge #263GROUP BYHAVINGDATA QUALITY

Find Orders with More Than One Payment Record

Return the orders that have two or more rows in the payments table.

The challenge runs on the SQLabHub practice database (SQLite) and uses the payments table. You can solve it in the browser, with nothing to install.

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The task

Find orders that have more than one payment record.

A correct answer returns 59 rows with one column: order_id.

Tables used in this challenge

payments 200 rows

id INTEGER (PK) · order_id INTEGER · user_id INTEGER · amount REAL · method TEXT · status TEXT · paid_at TEXT · transaction_id TEXT

First 3 rows of the payments table
idorder_iduser_idamountmethodstatuspaid_attransaction_id
146105921.62Gift Cardsuccess2022-05-18TXNHTN4GIK5L
2157371723.82Bank Transfersuccess2024-11-11TXNKJPIXJOFP
34536978.76Bank Transferrefunded2023-10-24TXNU2RW4MK1R

The first rows of each table are shown. See how all the tables relate on the database schema page.

Expected output

Expected output of the challenge
order_id
2
3
5
8
14

First 5 of 59 rows. This is the output of the model solution on the practice database. The task does not fix a sort order, so your rows may come back in a different order.

What you practise

  • Summarising many rows into one value with COUNT, SUM, AVG, MIN or MAX
  • Grouping rows with GROUP BY to get one result row per group
  • Filtering groups after aggregation with HAVING

Hint and approach

Try the challenge first. Open these only when you need them.

Show the hint

GROUP BY order_id HAVING COUNT

Show the step-by-step approach

How the model solution is built, in the order the database evaluates it:

  1. Read from the payments table.
  2. Group the rows by order_id, so the result has one row for each value.
  3. For each group, compute COUNT(*).
  4. Filter the groups in HAVING: keep those where COUNT(*) is greater than 1.

Model solution

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What to watch for

Group by the key and keep the groups with a count above 1. The same query finds duplicate emails, invoice numbers or any other value that should be unique.

  • HAVING. Conditions on single rows belong in WHERE and conditions on aggregates in HAVING. Putting a row filter in HAVING works, but it groups rows that are about to be thrown away.
  • GROUP BY. Every selected column must either be in GROUP BY or sit inside an aggregate. SQLite accepts other columns and returns an arbitrary value for them; most databases raise an error.
  • Aggregate functions. Aggregates skip NULLs. AVG divides by the number of non-NULL values and COUNT(column) can be smaller than COUNT(*).
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